1. Field of the Invention
This invention relates to an apparatus and method for the treatment of a flue gas, in particular for the reduction of nitrogen oxides within the flue gas. The apparatus includes a number of distributor tubes with nozzle openings for the injection of a reducing agent, in particular ammonia, whereby the distributor tubes are distributed in at least one cross-sectional plane of the flue gas duct and are each connected by means of a control valve with feed lines.
2. Description of the Prior Art
In such flue gas ducts of the prior art, each distributor tube extends over the entire width of the flue gas duct. Accordingly, when the control valve of the distributor tube is opened, reducing agent is injected over the entire width of the flue gas duct. Generally, to reduce concentrations of nitrogen oxides (NO.sub.x) in flue gas, the most commonly used process is selective catalytic reduction (SCR) on plate-shaped or grid-shaped catalyst bodies, in which ammonia (NH.sub.3)-sometimes premixed with air-or another reducing agent is added in measured quantities to the flue gas.
It has been shown that during the operation of a flue gas duct, the nitrogen oxides are not uniformly distributed in the flue gas, but that there are non-uniform nitrogen oxide concentrations over the cross section of the flue gas duct. As a rule, there are local fluctuations in the NO.sub.x concentrations, the flue gas velocities and the oxygen concentrations. Moreover, it should be recalled that in large combustion facilities, for example, the flue gas duct cross section can be up to 100 square meters. Therefore, a dosing apparatus is required which makes it possible to dose the locally required amount of reducing agent for such large flows of flue gas throughout the duct cross section.
If, as in some prior art systems, the reducing agent is injected uniformly into such "uneven spots" in the concentration, the result is an uneven catalyst action, since there is not one Mol of nitrogen oxide for one Mol of reducing agent in all areas.
A number of U.S. patents disclose systems wherein a reducing agent is utilized to remove nitrogen oxides from flue gas or exhaust gases with the aid of a catalyst. Some of these patents are directed to the physical construction of the catalyst and to other features intended for effective removal of the nitrogen oxides. Such U.S. patents include U.S. Pat. No. 4,672,052 entitled "Catalyst for Removal of Nitrogen Oxides from Exhaust Gases"; U.S. Pat. No. 4,695,559 entitled "Catalyst for the Selective Reduction of Nitrogen Oxides in Waste Gases and Process for the Manufacture of Such a Catalyst"; U.S. Pat. No. 4,720,376 entitled "Process for the Removal of Nitrogen Oxides and Soot from Exhaust Gases of Machines and Combustion Installations Burning Heavy Fuel Oil"; No. U.S. Pat. 4,721,699 entitled "Catalyst for the Separation of Nitrogen Oxides from Combustion Exhaust Gases"; U.S. Pat. No. 4,742,036 entitled "Catalyst Plate"; No. U.S. Pat. 4,758,410 entitled "Cartridge for Catalyst Plates"; and U.S. Pat. No. 4,760,047 entitled "Catalyst Block". These patents are incorporated by reference as if they were set forth herein in their entirety.